joist (n.) Look up joist at Dictionary.com
early 14c. (late 13c. in Anglo-Latin), from Old French giste "beam supporting a bridge" (Modern French gîte), noun use of fem. past participle of gesir "to lie," from Latin iacere "to lie, rest," related to iacere "to throw" (see jet (v.)). Notion is of wooden beam on which boards "lie down."